The present invention generally relates to monitoring, metering, protection and control of electrical systems, and more specifically, to an apparatus and method for estimating synchronized phasors at predetermined times referenced to an absolute time standard in an electrical system.
Knowledge of the state of an electrical network is important in order to recognize and understand disturbances in the electrical network, provide protection functionality, provide metering, monitor the electrical network, and determine control actions. This is especially true for critical electrical networks such as an electric power system or grid where energy is generated and transported from the generating facilities to locations and loads requiring the energy. Electrical power systems include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. The electric power systems also include various monitoring devices, control devices, metering devices, and protective devices (e.g. protective relays). In most cases, these devices are microprocessor-based or “intelligent” electronic devices (IEDs), that utilize phasors (i.e., a constant complex number representing a sinusoidal function of time) to perform their respective function(s). The phasors are derived from local or shared remote measurements sampled from currents and/or voltages of the electric power system.
Regardless of whether the phasors are derived from local or remote measurements, the accuracy of such measurements is of paramount importance when achieving a desired level of electrical power system performance. Such accuracy is predicated on both the accuracy of the measurement itself and the accuracy of the knowledge of the acquisition time of the measurement.
Acquisition time accuracy has been addressed via time-keeping systems that distribute highly accurate time, referenced to an absolute time standard, which have been used to drive the acquisition time of a voltage or current measurement. The absolute time standard typically includes one of the coordinated universal time (UTC) or international atomic time (TAI), distributed by the Global Positioning System (GPS) and then with a time protocol such as the Inter Range Instrumentation Group time code standard (IRIG) or over Ethernet. Systems employing absolute time reference schemes utilize voltage and/or current measurements that are sampled from analog voltage and/or current signal(s) with respect to the absolute time. As a result, both the measurement value and the time at which the measurement value is acquired can be obtained, processed, stored, and/or transported with high accuracy.
In applications such as power system state determination, it is desirable to sample voltage and/or current signals at many points across the electric power system at the same moment. In that case, using an absolute time standard, the voltages and/or current signals are sampled at a coordinated time instant to allow synchronization of the acquisition across the network. The resulting voltage and/or current measurements are processed to form “synchronized phasors” which are then utilized to enable new applications for the monitoring, metering, protection, and control of the electric power generation, transmission, and distribution network of the electric power system. The synchronized phasors may also be stored for subsequent use in analyzing a fault or other anomalous electrical power system condition.
Real-time monitoring of the present state of the electrical power system is often accomplished using a state-estimation algorithm. In general, the state-estimation algorithm utilizes measured voltage and power measurements that are collected from monitored points or nodes in the electric power system. The measured quantities are then used to estimate the state of the electric power system. One inherent limitation of state-estimation algorithms however is the time delay introduced as a result of the estimation of the power system state. It is therefore desirable to minimize the delay due to determination of the estimated state values, as well as any delay between the determination of the estimated state values and any subsequent control action (i.e. a control latency).
An electric power system utilizing the synchronized phasors can aid in reducing the control latency inherent in state-estimation algorithms. Because each synchronized phasor represents a present state value of the power system, derivation of estimated state values is not required, and the mechanisms which determine the control action are able to act more quickly. A further application for the use of synchronized phasors includes the dynamic or continuous recording of the variable electric power system magnitude and phase angle. Utilizing such a dynamic recording enables subsequent analysis of changes in the electric power system due to, for example, opening or closing a line, equilibrium differences between generation and load, or unstable power swings. A number of types of protection functions such as those found in protective relays may also benefit from the use of synchronized phasors. For example, synchronized phasors may be used to improve detection of the loss of electric power system synchronization when generators begin operation at different speeds. When the loss of synchronization is detected, a remedial action scheme can be quickly employed to “island” a portion(s) of the electric power system or shed a load.
As mentioned above, phasors are calculated from stepped-down analog voltage and current signals of the electric power system. When received from respective voltage and current transformers, the analog voltage and/or current signals are filtered, sampled with respect to the local power system frequency (e.g., 60 Hz) and processed to form phasors suitable for use by a microprocessor or other intelligent device. Synchronous phasors are similarly calculated except that they are sampled with respect to an absolute time standard and derived with respect to a coordinated time instant.
For example, one method of computing the synchronized phasor is described in U.S. Pat. Nos. 6,845,333 and 6,662,124 assigned to Schweitzer Engineering Laboratories, Inc. In one embodiment, the absolute time reference is generated via GPS, transmitting time and location information that is used by local receivers of the intelligent electronic devices. The acquisition may be based on deriving each sample instant from the absolute time reference, and recording the time at which the sample is taken.
To be meaningful, a synchronized phasor or phasor quantity must be referenced to a standard to allow correlation to other synchronized phasors even when the synchronized phasors are acquired from multiple electric power system locations with varying states and frequencies. One method known in the art that may be used to reference a synchronized phasor includes correlating an associated sampled voltage or current signal to a phasor with a frequency equal to the nominal power system frequency and with a predetermined phase that has been set in all of the IEDs or measuring devices. Alternatively, a single analog quantity can be chosen as the reference for all IEDs communicating and sharing the synchronized phasors. For example, the frequency at one point in the distribution scheme is sometimes chosen as a reference for all devices computing synchronized phasors. In some cases a feature of a reference signal, such as a zero crossing, can provide the reference for the phase value.